Multiband antennas for wireless voice and data communications are known. For example, common frequency bands for GSM services include GSM900 and GSM1800. A low band of frequencies in a multiband antenna may comprise a GSM900 band, which operates at 880-960 MHz. The low band may also include Digital Dividend spectrum, which operates at 790-862 MHz. Further, the low band may also cover the 700 MHz spectrum at 698-793 MHz.
A high band of a multiband antenna may comprise a GSM1800 band, which operates in the frequency range of 1710-1880 MHz. A high band may also include, for example, the UMTS band, which operates at 1920-2170 MHz. Additional bands may comprise LTE2.6, which operates at 2.5-2.7 GHz and WiMax, which operates at 3.4-3.8 GHz.
When a dipole element is employed as a radiating element, it is common to design the dipole so that its first resonant frequency is in the desired frequency band. To achieve this, the dipole arms are about one quarter wavelength, and the two dipole arms together are about one half the wavelength of the desired band. These are commonly known as “half-wave” dipoles. Half wave dipoles are fairly low impedance, typically in the range of 73-75.52.
However, in multiband antennas, the radiation patterns for a lower frequency band can be distorted by resonances that develop in radiating elements that are designed to radiate at a higher frequency band, typically 2 to 3 times higher in frequency. For example, the GSM1800 band is approximately twice the frequency of the GSM900 band.
There are two modes of distortion that are typically seen, Common Mode resonance and Differential Mode resonance. Common Mode (CM) resonance occurs when the entire higher band radiating structure resonates as if it were a one quarter wave monopole. Since the vertical structure of the radiator (the “feed board”) is often one quarter wavelength long at the higher band frequency and the dipole arms are also one quarter wavelength long at the higher band frequency, this total structure is roughly one half wavelength long at the higher band frequency. Where the higher band is about double the frequency of the lower band, because wavelength is inversely proportional to frequency, the total high band structure will be roughly one quarter wavelength long at a lower band frequency. Differential mode occurs when each half of the dipole structure, or two halves of orthogonally-polarized higher frequency radiating elements, resonate against one another.
One known approach for reducing CM resonance is to adjust the dimensions of the higher band radiator such that the CM resonance is moved either above or below the lower band operating range. For example, one proposed method for retuning the CM resonance is to use a “moat”. See, for example, U.S. patent application Ser. No. 14/479,102, the disclosure of which is incorporated by reference. A hole is cut into the reflector around the vertical section of the radiating element (the “feedboard”). A conductive well is inserted into the hole and the feedboard is extended to the bottom of the well. This lengthens the feedboard, which moves the CM resonance lower and out of band, while at the same time keeping the dipole arms approximately one quarter wavelength above the reflector. This approach, however, entails extra complexity and manufacturing cost.